Elliptical Galaxies

Elliptical galaxies have the shape of an oblate spheroid. They appear as luminous elliptical disks. The distribution of the light is smooth and the intensity falls off from the center as $I(r) \propto r^{-1/4}$, where I(r) is the intensity and r is the distance from the center of the galaxy. Elliptical galaxies are classified based on the elongation of their apparent projected images. If a and bare the major and minor axes of the apparent ellipse, than 10(a-b)/awould be the expression of the observed ellipticity. The classification is based on the apparent elongation because the true orientation and ellipticity of the galaxies are not known (Zeilik, Gregory & Smith 1992). The classification of elliptical galaxies ranges from E0 to E7. The most spherical looking ones (with apparent ellipticity of 0) are classified E0, whereas the most flattened ones (with maximum apparent ellipticity) are designated E7 (Fix 1995).